International Conference on Environment 2008 (ICENV 2008)
ID 000146
THE IMPORTANCE OF NOVEL SRU TECHNIQUES ON
ALLEVIATION OF THE ENVIRONMENTAL ISSUES IN
IRANIAN OIL AND GAS REFINING INDUSTRIES
A. SHAHSAVAND 1, A. GARMROUDI,
CHEMICAL ENGINEERING DEPT.,FACULTY OF ENGINEERING
FERDOWSI UNIVERSITY OF MASHHAD,
MASHHAD, P.O. BOX 91775-1111
I.R. IRAN
E-mail: shahsavand@um.ac.ir
ABSTRACT
Islamic Republic of Iran is the second largest natural gas (NG) owner in the world (after Russia),
possessing around 16% of the global NG reserves (near 975 TCF). It also occupies the third rank
among the greatest oil owners (after Saudi Arabia and Canada) by controlling about 11 percent of
the global crud oil reserves (around 136 billion barrels of recoverable oil). Since most of these
oil and gases are contaminated with hydrogen sulfide, therefore the production of near five
thousand tones of elemental sulfur per day from hydrogen sulfide plays an essential role in
Iranian natural gas and oil industry. All existing sulfur recovery units (SRU's) are using various
varieties of Claus processes. Such relatively old units require expensive catalysts while providing
low recovery efficiencies (as low as 90 percent or even less). Furthermore, the Iranian officials
plan to construct more than 5 SRUs in the next 10 years to produce around 4000 tons of
elemental sulfur per day from the south Pars sour gas reserve (the largest NG reservoir in the
entire world).
This article investigates various environmental aspects of conventional Claus processes and
emphasizes on the importance of using novel sulfur recovery technologies such as biotechnology
(microbial degradation) and thermal decomposition of hydrogen sulfide (known as thermolysis)
to avoid further air pollution of the Persian gulf region. Fortunately, these new technologies not
only provide higher sulfur recovery efficiencies (more than 99.4 percent) at relatively low
temperatures, but also they produce huge amounts of hydrogen gas which is one of the cleanest
sources of energy. Although, the purity of produced hydrogen still does not meet the allowable
required criteria, however, it is predicted that the new metallic membranes will provide sufficient
means for hydrogen purification in the next few years.
Keywords: Natural Gas; Hydrogen sulfide; Sulfur recovery; Claus, Thermolysis; microbial
degradation
INTRODUCTION
Table 1 presents the maximum production capacity of various Iranian sour gas refineries.
Twenty more gas refineries will be constructed in the next ten years. Each unit will eventually
produces 25MMSCMD sweet gas plus 40 thousand barrel condensate and 200 tonnes of
elemental sulphur per day. The total sweet gas production capacity will be increased to around
1000 MMMSCMD in 2015.
1
Corresponding author
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International Conference on Environment 2008 (ICENV 2008)
ID 000146
Table 1: Maximum production capacities of Iranian sour gas refineries (2007)
No .
1
2
3
4
5
6
7
Refinery
South pars (phases 1 to 5)
Fajre-Jam
Parsian
Khangiran
Bidboland
Dalan
Sarkhoon
Total
Sour gas capacity
(MMSCMD)
125
110
60
50
25
20
15
405
Furthermore, as it is shown in table 2, Iranian refineries distil around 2 million barrels per
day crude [2]. A large portion of the refined oil is contaminated with various sulphur compounds
such as mercaptans, disulfides and sulfonic acids. The hydro-treating (also known as unifining)
units of these refineries also produce several hundred tonnes of elemental sulphur per day.
Iranian oil and gas refineries produce more than one and a half million metric tonnes elemental
sulphur per year with the estimated price of about one billion dollar [1]. Around 40 percent of the
above figure belongs to Khangiran sour gas refinery which treats the sourest natural gas feed
among all Iranian refineries. The H2S concentration of Khangiran sour gas refinery feed is as
high as 3.5 percent by volume. The total sulphur production is anticipated to be more than three
million metric tonnes per year in 2015.
Table 1: Maximum production capacities of Iranian crude oil refineries (2007)
No .
1
2
3
4
5
6
7
8
9
Refinery
Capacity (tbpd)
450,000
250,000
280,000
120,000
320,000
180,000
50,000
25,000
25,000
1,700,000
Abadan
Tehran
Isfahan
Tabriz
Bandar Abbas
Arak
Shiraz
Kermanshah
Lavan
Total
SULFUR RECOVERY TECHNOLOGIES
Various sulfur recovery technologies such as liquid REDOX, Claus, Modified Claus and
super Claus processes are usually employed for production of elemental sulfur from acid gases in
oil and gas industries. The liquid REDOX processes such as Feld, Stretford, Sulferox and
LOCAT units use liquid phase reactions to initially reduce the hydrogen sulfide into elemental
sulfur and then oxidize the reduced catalysts or agents by oxygen or air. These processes are
usually suitable for small scale productions and are not very popular in oil and gas industries [2].
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A) TRADITIONAL CLAUS PROCESS
Not only all existing Iranian oil and gas refineries employ traditional Claus process to
convert hydrogen sulfide into elemental sulfur, however the same process is anticipated to be
used in the new twenty refineries planned to be constructed in the next ten years. Figure 1
illustrates the schematic diagram of typical two stage Claus sulfur recovery unit [3]. The entering
acid gas is diluted with sufficient amount of air to oxidize about one third of the entering
hydrogen sulfide. The dominant reactions in the combustion chambers are:
Acid
Gas
Feed
Waste
Heat
Boiler
Combustion
Chamber
(Furnace)
Sulfur
Catalytic
Condenser
Bed 1
Liquid
Sulfur
Direct
Reheat
Flare
Sulfur
Catalytic
Sulfur
Condenser
Bed 2
Condenser
TGC
Liquid
Sulfur
Liquid
Sulfur
Figure 1: Typical Claus sulfur recovery unit.
H 2 S  3 O2  SO2  H 2 O
2
H   760 kj
2 H 2 S  SO2  3 S 2  2 H 2 O
2
H   760 kj
2H 2 S  S 2  2H 2
H   20 kj
gmole
gmole
gmole
(1)
(2)
(3)
The furnace normally operates at combustion chamber temperature ranging from 980 to
1540° C with pressure rarely higher than 70 kPa. The recovery efficiency of first sulfur condenser
is about 60 to 70%. Depending on the furnace temperature, about 30-60 percent of the elemental
sulfur produced in combustion chamber is due to the third reaction. Figure 2 shows that the
pyrolysis of hydrogen sulfide into hydrogen and elemental sulfur start around 500°C and rapidly
increases with increasing the furnace temperature. At around 1500°C, more than 60 percent of the
entering H2S will decompose to sulfur and hydrogen, provided that sufficient heat is produced by
the first two exothermic reactions.
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International Conference on Environment 2008 (ICENV 2008)
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Figure 2: Dissociation efficiency of hydrogen sulfide versus temperature.
Liquid sulfurs from all condensers run through seal legs into a pit in which they cool
down and turn to solid sulfur with a purity of approximately 98.5 weight percent [2]. The overall
efficiency of the entire Claus process (using 2 or more catalytic converters) strongly depends on
the hydrogen sulfide content of acid gas entering the sulfur recovery unit. For example a two or
three stage plant could have a recovery efficiency of 95% for a 90% H2S stream, 93% for 50%
H2S and 90% for 15% H2S [3].
As it was mentioned earlier, the optimal temperature of Claus process combustion
chamber is about 1000 -1500°C . The pyrolysis of hydrogen sulfide rapidly increases with
increasing the temperature. The high contamination of most Iranian acid gases with inert
materials such as carbon dioxide (separated also from sour gases) result to sever combustion
chamber temperature drop of as low as 850°C. For this reason, the overall efficiencies of Iranian
sulfur recovery units are usually around 90% (or even less).
In other words, about 150 thousand tones per year of sulfur are directly exhausted to
atmosphere. Evidently, most of these sulfurs are burned in refineries flares prior to discharging
into atmosphere. About one thousand tones per day of sulfur dioxide which enter the Iranian
atmosphere have enormous environmental adverse impacts such as acid rains which can lead to
sever corrosion problems and devastating environmental issues. In order to increase the sulfur
recovery efficiency we may resort to one or a combination of the following remedies:
 Enriching the hydrogen sulfide content of the acid gas feeds by using membrane or
alkanol-amines absorption processes.
 Enrichment of the combustion air injected to Claus furnace by using membrane or
pressure swing adsorption processes.
 Using super clause process which produces part of elemental sulfur via following partial
oxidation reaction.
2H 2 S  O2 Catalyst

 S 2  2H 2 O
(4)
 A combined process maybe used to increase the overall recovery efficiency. For example,
Claus or super Claus process maybe coupled with tail gas conditioning (TGC) unit to
remove about 99% of sulfur from original acid gas.
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International Conference on Environment 2008 (ICENV 2008)
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 In another approach, we may use different scenarios or reactions to produce sulfur from
cid gases. For example we may biologically oxidize H2S to make elemental sulfur [4] or
directly pyrolysis the hydrogen sulfide to sulfur and Hydrogen. Huge amounts of
hydrogen gas produced in this process are extremely valuable as the cleanest fuel [5].
B) BIOLOGICAL SULFUR RECOVERY UNITS
Certain biological species such as Thiobacillus , Thiothrix and Beggiatoa can produce
elemental sulfur from acid gases containing sufficiently large hydrogen sulfide concentrations.
The optimum pH of the culture environment varies greatly from 1.5 - 2.5 for Thiobacillus up to
7.5 - 12 for other types of bacteria’s such as Beggiatoa. Although these bacteria’s can produce
elemental sulfur from acid gases via the following reactions:
Micoorganism
2 H 2 S  O2 

 2 S o  2 H 2 O
(5)
2 H 2 S  O2 
 2 S  2 H 2 O
(6)
Micoorganism
o
However, the produced sulfur is usually contaminated with the dead bodies of
microorganisms and it can not be used as raw material for most industry applications. On the
other hand, such contaminated sulfurs can be used in limited scales as soil fertilizes. Furthermore,
the production costs of biological sulfur recovery techniques usually lie between $0.08 to $0.12
per 1000 SCF sour gas[6]. These operational costs are slightly higher than conventional sulfur
recovery units. The main advantage of this process is its environmentally friendly behavior. The
elemental sulfur is usually produced directly from acid gases dissolved in some solutions using
certain types of bacteria's. Therefore no sulfur dioxide will be exhausted to atmosphere by
biological technique.
C) DIRECT THERMOLYSIS OF HYDROGEN SULFIDE
In 1975, Raymont illustrated in his PhD thesis that the direct thermolysis of hydrogen
sulfide at very high temperatures (of about 1500°C) can be used to produce elemental sulfur and
hydrogen gas [7]. In 1993 Edlund and Pledger used a multilayer catalytic hydrogen permeable
composite membrane reactor to produce hydrogen and elemental sulfur from direct
decomposition of hydrogen sulfide. The membrane contains a platinum coated layer on the feed
side of the membrane. This layer resists irreversible chemical attack by H2S at relatively high
temperatures. They reported that a complete conversion of 99.4% was a achieved in a laboratory
scaled membrane [8]. No sulfur dioxides were also formed in this reaction.
Figure 3 shows the schematic diagram of a typical thermolysis sulfur recovery unit.
Instead of burning hydrogen sulfide, it can be dissociated at 400-700K in the presence of certain
porous catalysts coated on the surface of palladium membrane. The hydrogen gas permeates
through the membrane and shifts the following equilibrium decomposition reaction to the right.
Heat and Catalyst
2 H 2 S 

 S 2  2 H 2
(7)
The required heat of reaction can be prepared by burning sufficient amount of fuels such
as natural gas. Evidently using economized heat exchanger systems can preheat the feed by hot
product streams and reduce the fuel consumption rate (and eventually reduces the global warming
risks). Composite hydrogen transport membranes are made of supporting metals and metal alloys
which have high hydrogen permeability, but are either too thin to be self supporting or too weak
to resist differential pressures across the membrane.
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Hydrogen
Catalytic
Sulfur
Acid
Gas
Feed
Compressor
Furnace
Membrane
Condenser
Fuel
Liquid
Sulfur
Sulfur
Flare
TGC
Condenser
Liquid
Sulfur
Figure 3: Schematic diagram of a catalytic thermolysis process for sulfur recovery units
Support materials are chosen to be lattice matched to the permeable metals and metal
alloys. Hydrogen-permeable membranes include those in which the pores of a porous support
matrix are blocked by hydrogen-permeable metals and metal alloys. Preferred metals with high
permeability for hydrogen include vanadium, niobium, tantalum, zirconium, palladium, and
alloys thereof.
RESULTS AND DISCUSSIONS
Figure 4 shows the dependencies of measured H2 permeate rates across the palladium
membrane versus temperature at different hydrogen partial pressures. Increasing the temperature,
results higher flux rates due to larger diffusivity of the hydrogen gas. Also, Increasing the
hydrogen partial pressure increases the driving force and produces larger flux rates.
Figure 4: Membrane permeate rates versus temperature at different H2 partial pressures
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International Conference on Environment 2008 (ICENV 2008)
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Table 3 shows the improvements achieved on various aspects of palladium membranes in
the few recent years and the targets anticipated for the near future (2010). It is believed that
obtaining the forecasted values will enable engineers to construct the first industrial sulfur
recovery unit based on direct thermolysis process. Evidently, the optimal operating temperature is
already achieved and the durability is somewhat satisfactory (50,000 hrs ≈ 5.8 years) at the
present time, however, the permeate flux still should be improved considerably and the
membrane cost must also be reduced below 100 dollars for each square feet of the metallic
membrane.
Table 3: Current characteristics and forecasted requirements of metallic membranes
Parameters
Flux
Cost
Durability
Operating temperature
Parasitic power
Units
SCFH/ft2
$/ft2
hrs
°C
kWh/1000SCFH
2003
60
150 - 200
< 1,000
300 - 600
3.2
2005
100
100 - 150
50,000
300 - 600
3.0
2010
200
< 100
100,000
300 - 600
2.8
Table 4 illustrates the environmental achievements of such novel technology compared
with the traditional Claus processes used currently in Iranian oil and gas refining industries.
Hydrogen production is a large and growing industry. Globally, some 50 million metric tons of
hydrogen, equal to about 170 million tons of oil equivalent, were produced in 2004. The growth
rate is around 10% per year. As of 2005, the economic value of all hydrogen produced worldwide
is about $135 billion per year [9]. Using the above figures, the price of each metric tone of
hydrogen will be about 2500 dollars.
Table 4: Comparison of thermolysis process with traditional Claus unit.
Parameter
Recovery
Exhausted sulfur dioxide
Produced hydrogen
Hydrogen Price
Equivalent methane saving
Required methane (for heating acid gas feed
from 300K to 600K)
Net methane saving
Less CO2 exhausted to atmosphere
Sulfur savings due to higher conversion
Units
percent
MTones per year
MTones per year
MM Dollars/ year
MTones per year
MTones per year
Thermolysis
99.4
18
93
232
210
0.01
Claus
< 90
300
0
0
0
0
MTones per year
MTones per year
MM Dollars/ year
210
578
100
0
0
0
As it can be seen, the produced hydrogen can save about 210 thousand tones of CH 4 per
year which greatly reduces the global warming risks. Furthermore, the new thermolysis process
will eventually leads to about 600 thousand tones less carbon dioxide exhausted to atmosphere.
Evidently, this amount of saving will have great impact on the Middle East region environment
and perhaps the entire planet global warming problem.
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International Conference on Environment 2008 (ICENV 2008)
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CONCLUSION
Iranian oil and gas industries discharge about 300 thousand metric tones sulfur dioxide
per annum for production of 1.5 million metric tones of elemental of sulfur via traditional Claus
process. Employment of novel sulfur recovery techniques such as bacteria or thermal dissociation
of hydrogen sulfide can drastically reduce various sulfur oxides exhaustion rate to atmosphere.
The produced sulfur via biological method is usually contaminated with the dead bodies of
microorganisms and it can not be used as raw material for most industry applications. On the
other hand, direct thermolysis of hydrogen sulfide in a catalytic membrane can produce pure
sulfur with a conversion rate of as high as 99.4%. Using this novel process, will produce around
93 thousand metric tones extra hydrogen which eventually results to about 600 tones less
exhaustion of CO2 to atmosphere. Furthermore, over 330 million dollars will be saved per year
for extra sulfur and hydrogen produced.
AKNOWLEDGEMENTS
The authors wish to acknowledge the valuable contribution of National Iranian Gas
Company (NIGC) officials for providing necessary information. We also appreciate Ferdowsi
university of Mashhad for their financial support.
REFERENCES
[1] U.S. Geological Survey, Mineral Commodity Summaries, January 2006 (available on :
http://minerals.usgs.gov/minerals/pubs/commodity/sulfur/sulfumcs06.pdf)
[2] Adelzadeh, M.R. Fundamentals and calculations of petroleum refining engineering, Ava
book, 2007, pp 315-320.
[3] AP 42, Fifth Edition, Volume I, Chapter 8: Inorganic Chemical Industry, Section 8.13: Sulfur
recovery,1993 (available on :http://www.epa.gov/ttn/chief/ap42/ch08/index.html)
[4] United States Patent 7351392 , Process for the high recovery efficiency of sulfur from an acid
gas stream, 2008 (available on: http://www.freepatentsonline.com/7351392.html)
[5] Harvey W. S., Davidson J. H., Fletcher E. A., Thermolysis of hydrogen sulfide in the
temperature range 1350-1600 K, Industrial & engineering chemistry research,
37(6)1998, pp. 2323-2332.
[6] Basu, R. , Clausen, E. C. , Gaddy, J. L. Air Pollution Control Biological conversion of
hydrogen sulfide into elemental sulfur, Environmental Progress, 15(4) , 2006, pp 234 – 238.
[7] M.E.D. Raymont, Role of hydrogen in Claus plants, Hydrocarbon Processing, 177-179
(1975).
[8] Edlund, D. J. , Pledger, W. A. Thermolysis of hydrogen sulfide in a metal-membrane reactor,
J. of Membrane Science, 77, 1993, pp 255-264.
[9] Hydrogen economy, From Wikipedia, the free encyclopedia (available on :
http://en.wikipedia.org/wiki/Hydrogen_economy)
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